The present invention relates generally to swash-plate type piston pump motors. More particularly, the invention relates to piston pump motors of the type wherein translational movements in the axial direction of pistons actuated by hydraulic pressure are converted into rotational movement by utilizing a swash plate.
In a piston pump motor of this type, a cylinder block or rotor is coaxially mounted on and coupled by a spline to an output shaft or rotating shaft. Within this cylinder rotor are formed cylinder bores in which respective pistons of reciprocating type are slidably fitted. A swash-plate part is fixedly mounted with an inclined attitude relative to the output shaft. One end of each piston is in abutting contact, by way of a shoe functioning as a universal joint, with the swash-plate part. The other end of the piston faces a hydraulic pressure chamber of the corresponding cylinder bore.
Hydraulic pressure is supplied from the outside into each hydraulic pressure chamber to actuate the piston axially toward the swash-plate part. This translational motion in the axial direction is converted into rotation of the cylinder rotor and therefore of the output shaft. Furthermore, during operation each shoe slides in rubbing contact with the swash-plate part as it presses thereagainst. In order to obtain smooth sliding action between these moving parts, hydraulic oil is supplied from the hydraulic pressure chamber, through a throttle hydraulic fluid passage, to the pressing sliding surfaces of these moving parts.
In a conventional piston pump motor of this construction, a pressure arising from pressure fluctuations in the hydraulic pressure chambers is transmitted from the cylinder rotor to one of the end walls of a housing assembly enclosing all working parts. In a state of offset phase relative to this pressure, a pressure corresponding to fluctuations in the pressure of the pressing sliding surfaces is transmitted from the swash-plate part to the other end wall of the housing assembly. In addition, as a consequence of factors such as an inertial effect, loads arising from the reciprocating motions of the pistons are transmitted with mutually different timing to both end walls of the housing assembly. These pressures and loads having timing lags are propagated or transmitted from both end walls to the entire housing. Consequently, the housing assembly undergoes out-of-plane vibration and thereby generates noise.
More specifically, a piston pump motor is ordinarily provided with a flange for coupling to a relatively large speed-changing mechanism. This flange extends outwardly in radial directions from one end part of the peripheral wall or barrel part of the housing assembly. The annular mounting seat of the outer peripheral edge part of this flange is adapted to be fixed to the case of the speed-changing mechanism. Consequently, this flange is of a construction which readily undergoes out-of-plane vibration. When pressures and loads causing vibrations are transmitted to the housing assembly as described above, the mounting flange undergoes an out-of-plane vibration of large magnitude, and noise is generated.
With the aim of producing a construction for preventing noise of this character, the additional provision of a vibration-suppressing steel plate at the swash-plate part has been proposed in Japanese Utility Model Application Laid-Open Publication No. 62-98779. This vibration-suppressing steel plate comprises two steel plates and a special viscoelastic resin sandwiched therebetween. By the use of the device disclosed in this publication, the transmission of vibration from the swash-plate side to the housing can be suppressed by the vibration-suppressing steel plate.
Another noise-preventing construction featuring improvements in the housing construction is disclosed in Japanese Patent Application Laid-Open Publication No. 62-174579. In this device, the thickness of the end wall on the side opposite the swash plate part of the housing is made extremely large thereby to increase the rigidity. The thick end wall and the swash plate supporting part are connected by long bolts.
In the construction disclosed in the above cited first reference, Japanese Utility Model Application Laid-Open Publication No. 62-98779, the vibration-suppressing steel plate is a part of special fabrication, which unavoidably increases the production cost.
On the other hand, in the construction proposed in the above cited second reference, Japanese Patent Application Laid-Open Publication No. 62-174579, and extremely thick end plate is used, whereby the dimension in the axial direction and the weight of the entire device increase.
Furthermore, in each of the constructions of the above cited references, the generation of vibration to some degree in the housing is unavoidable. This vibration becomes noise, which cannot be prevented from being transmitted to the outside.